Inkjet printing systems typically use thermal energy that is selectively produced by resistors located in ink-filled capillary channels near channel-terminating nozzles or orifices. The selectively applied thermal energy momentarily vaporizes the ink and forms bubbles. The bubbles expand, expel an ink droplet and propel it toward a recording medium, e.g. a sheet of paper.
Such a printing system may be incorporated in either a carriage-type printer or a page-width type printer. The carriage-type printer generally has a relatively small print head containing the ink channels and nozzles. The print head is usually connected to a disposable ink supply cartridge. The combination of the print head and cartridge assembly is reciprocated back and forth to print one line of information at a time on the recording medium. After each line is printed, the paper or other recording medium is stepped a distance equal to the height of the printed line, so that the next line can be printed. The procedure is repeated until the entire page is printed.
In contrast, the page-width printer has a stationary print head with a length equal to, or greater than, the width of the print medium. The paper, or other print medium, is moved past the page-width print head in a direction normal to the print head length and at a constant speed during the printing process. High-speed, page-width printers are used in a variety of applications. For example, Point-of-Sale (POS) printers are used to produce receipts at most retail stores.
Inkjet printing systems, however, are prone to several problems that adversely affect the quality and performance of the printing. Among these problems are (1) clogging of the print head nozzle caused by ink drying therein (which may occur due to a period of non-use), (2) adherence of dust to the face of the nozzle due to the moisture of the fluid ink around the nozzle, (3) leakage of ink from the nozzle, (4) bubbles and dust taken into the print head nozzle as a result of external causes such as vibration imparted to the print head and environmental changes occurring around the print head, and, finally, (5) contamination of the print head nozzles when the print head is not in use, caused by, for example, non-collapsing air bubbles. These problems, if not corrected, result in nozzles that cannot eject ink properly and the resulting print quality is degraded.
Several approaches have been proposed to address these problems associated with inkjet printing systems. Most of the proposals include a service station at one end of the printer to clean the print nozzles with a wiper element. However, these servicing systems locate the service station adjacent to the paper conveying system and require the print head to be moved alongside the paper conveying system. Consequently, it is difficult to use such servicing systems with a stationary page-width print head. Furthermore, most page-width stationary print heads are used with a continuous paper conveying system (e.g., a platen) that may block access between a service station and the stationary print head. An example of an inkjet print head servicing mechanism that includes a wiper element is shown in U.S. Pat. No. 5,051,761, which is hereby incorporated by reference in its entirety.
In addition, the wiping elements of typical print head service stations often become dirty themselves. After many servicing operations, it is not uncommon for the servicing wiper to become as dirty as the print head, and the wiping function no longer produces any useful result.